1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus capable of maintaining the stabilized lighting of discharge lamps.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a conventional metal halide lamp lighting apparatus.
A lighting apparatus 500 shown in FIG. 1 lights a metal halide lamp 505 by supplying DC voltage that was obtained by the full-wave rectification and smoothing of AC voltage from an AC power source 501 to a load lamp starting circuit 504.
At this time, electric power to be supplied is regulated to a desired constant level by a choke coil 506 and a switching transistor 507 provided at the electric power supply line to the starting circuit 504.
That is, to regulate electric power to a constant level, a voltage value of the starting circuit 504 is first measured by dividing the terminal voltage of the starting circuit 504 by a load voltage detecting resistor 508 that is connected between both terminals of the starting circuit 504. Further, load current is obtained from the terminal voltage of a load current detecting resistor 509 provided at the minus terminal of the starting circuit 504. Then, from these voltage values of the starting circuit 504 and load current of the load current detecting resistor 509, present load consumption power is obtained by an electric power detecting circuit 511. This load power consumption is fed back to a PWM control IC 512. According to this fed back load consumption power, the base voltage of the switching transistor 507 is controlled by the PWM control IC 512. When its base voltage is controlled, the switching transistor 507 is switched so as to maintain the supply power to the metal halide lamp 505 at a constant level.
The electric power detecting circuit 511 secures the electric power using the choke coil as a transformer and the PWM control IC 512 also secures the electric power from the electric power supply line.
In computing electric power in the electric power detecting circuit 511, an analog multiplier is used but as accuracy of electric power computation is not sufficient, a constant electric power control is insufficient. So, it is desirable to compute electric power precisely using a microcomputer.
However, even when using an electric power detecting circuit employing a microcomputer instead of the electric power detecting circuit 511, there is such a problem as described below. That is, if an electric power detecting circuit using a microcomputer is connected to the secondary side of the choke coil 506 likewise the electric power detecting circuit 511 shown in FIG. 1, GND (Ground) of the microcomputer is not stabilized due to the switching operation of the switching transistor 507 and circulating current by the choke coil 506 and therefore, the operation of the microcomputer also is not stabilized.
So, it is desirable to control electric power supplied to the metal halide lamp 505 at a constant level by detecting voltage and current of the electric power supply line by connecting an electric power detecting circuit using a microcomputer to the AC power source 501 side of the electric power supply line from the switching transistor 507 and the choke coil 506.
For instance, to control the electric power at a constant level by detecting the power consumption of the metal halide lamp 505 by detecting the voltage and current of the electric power supply line without measuring the voltage of the metal halide lamp 505 as shown above, the voltage of the metal halide lamp 505 does not become constant and such a problem as shown below is produced.
That is, if equivalent resistance of the metal halide lamp 505 is low, abnormally large current flows to a load side and as a power loss is proportioned to a square of resistance value.times.current, the power loss tends to become extremely large. And the power loss is consumed in the switching transistor 507, diode 510, wiring, etc. highly heating them and such a deficiency as the functional stop or damage of elements will result.
On the contrary, if equivalent resistance of the metal halide lamp 505 is high, abnormally high voltage may be applied continuously. At this time, there will be such a problem that leak current will increase or safety will drop if an electric leakage is taken place.
Further, a technology to turn off a discharge lamp lighting circuit by detecting an abnormal state of a discharge lamp was disclosed in the Japanese Patent Publication of Unexamined Patent Application No. 6-20781. That is, threshold values of upper and lower limits for the lamp voltage of discharge lamps of cars are set and if a measured value of lamp voltage exceeds the upper limit threshold value or drops to below the lower limit threshold value after a prescribed time delay, such abnormal state that a discharge lamp is in the open state or in the shorted state is detected and based on the result of this detection, the discharge lamp lighting circuit is turn off.
However, lamp voltage of a high-pressure discharge lamp has such a character that the low voltage state continues for a while immediately after starting the lighting and thereafter, it rises to a rated lamp voltage. Because of such the character of the lamp voltage to vary in two steps, only by simply judging whether the lamp voltage falls below the lower limit threshold value as disclosed in the above mentioned Japanese Patent Publication of Unexamined Patent Application No. 6-20781, the abnormal state and the normal state of the lamp voltage cannot be fully discriminated. Therefore, there is a problem that an abnormal state of too low lamp voltage cannot be surely detected.
Further, for instance, in case of a fluorescent lamp, it was so far urged to exchange a lamp if the ends of a lamp tube are blackened or a lamp begins to flicker. Or, by setting an operating time of a lamp and conducting the maintenance work periodically, the lamp life was managed by exchanging a lamp before its service life was over.
However, a high-pressure discharge lamp has become widely in use by such business machines as OHP (Over Head Projector), projection TV, etc. in recent years.
So, if a high-pressure discharge lamp was burnt out, business and life are largely crippled and yet it is very troublesome to manage operating times of high-pressure discharge lamps and exchange them before their service lives are over. In addition, as no spare of expensive high-pressure discharge lamp is always reserved, such a problem comes out increasingly that business and lives are largely crippled as the life of high-pressure discharge lamp was suddenly exhausted.
Further, as slow leakage from a high-pressure discharge lamp is not easily detected, there is a problem that an abnormal state resulting from this slow leakage cannot be perceived.
In addition, if an interelectrode distance of a high-pressure discharge lamp is short, high lamp current flows continuously after lighting the lamp, heating the electrodes extremely and stress is accumulated in the sealed root portion of the electrodes and cracks may possibly be produced.